This invention relates generally to apparatus for felling trees and more particularly to an improved hydraulic circuit for controlling the movement of a cutter that forms part of the tree harvesting apparatus.
In recent years, considerable attention has been devoted to production of machines for the logging industry which can grasp the tree and sever the tree close to the ground. One type of such machine is disclosed in U.S. Pat. No. 3,575,222, which is assigned to the assignee of the present invention. In this type of machine, the severing is performed by a pair of blades that are moved by a single fluid ram. This particular machine is extremely useful under certain tree harvesting conditions and has found a remarkable degree of commercial success in certain areas of the world. However, under certain conditions, the shear-type of cutter has been found to split or shatter the wood adjacent the cut. This problem is not of any significant concern when the trees are to be used to produce pulp but results in substantial waste when the trees are used for the production of lumber or veneer because the damaged wood must be trimmed before the tree is finally processed.
Quite recently, another type of tree harvesting apparatus has been developed which utilizes a rotary cutter that operates somewhat like an elongated milling cutter. U.S. Pat. No. 3,902,538 and Canadian Pat. No. 960,549 disclose this type of cutter which can be advanced through a tree to cut chips from across the grain without crushing or splitting the fibers.
The last mentioned United States patent also discloses a hydraulic circuit for controlling the pivotal movement of the cutter as a function of the rotational resistance encountered by the cutter. However, it has been found that the hydraulic circuit disclosed in this patent is extremely difficult to accurately adjust for the proper operating conditions. In the hydraulic circuit disclosed in this patent, the speed of movement of the frame supporting the cutter is controlled by a rack and gear type flow-control valve which is adapted to be opened and closed by a spring bias one-way cylinder and piston rod arrangement.
In the arrangement disclosed in the above mentioned patent, the flow-control valve must be accurately "matched" to the remainder of the system and the adjustment thereof is extremely critical in order to provide a satisfactory operation. Also, the connection between the hydraulic motor driving the cutter and the hydraulic circuit for the fluid ram moving the frame supporting the cutter is what is commonly referred to as a mechanical connection, which again requires accurate alignment and adjustment of the various parts.
Another problem encountered with the drive system disclosed in the above mentioned United States patent is the fact that it cannot provide maximum utilization of available horse power for all the resistances that may be encountered by the cutter. This means that the feed speed is slower than necessary under many cutting conditions.